The purple membrane of Halobacterium halobium is a photon activated photosynthetic membrane composed of lipids and a well characterized protein (bacterio-opsin) complexed with the chromophore retinal. The structure and membrane physiology of the purple membrane has received considerable attention in the past ten years. Genetic analyses of this membrane have only been underway for the past two years. The gene for bacterio-opsin (bop) has been cloned and numerous spontaneous mutants have been characterized. It is clear that the synthesis of purple membrane is regulated by environmental factors such as light and oxygen tension but the genetic basis for regulation has been difficult to track. Our discovery of a gene (brp) associated with the expression of the bop gene has provided the first insight into a possible regulatory mechanism. We propose to investigate the role of the brp gene relative to the synthesis of purple membrane under a variety of culture conditions which suppress or enhance purple membrane synthesis. We also propose to develop a "genetic exchange" system for H. halobium. We will isolate and construct genetically marked lines of H. halobium which can be used to develop an efficient DNA transfer system such as, transformation, transduction or conjugation. Once available, a system of this type can be used to investigate the regulation of purple membrane synthesis by such classic approaches as complementation and gene fusion. In addition, a transformation and shuttle vector system is essential for the long-term goal of providing interesting site directed mutant forms of bacterio-opsin for structure and function studies. Of equal importance will be the contributions these studies will make to the virgin field of archaebacterial genetics.